In these devices, the ventricular stimulation frequency is variable, either by ensuring by the ventricle stimulation occurs in response to the atrial rate, or according to a parameter collected by a sensor. In the latter case, the known sensors are generally selected from among three types:
(1) A sensor of effort, which is a sensor measuring a parameter that is preponderantly physiological, and generally the measurement of the minute-ventilation, denominated a “sensor MV”, or the oxygen saturation in blood, or the temperature, etc. Such a sensor provides an adequate representation of the metabolic needs of the patient, according to whether the patient is at rest, in exercise, in recovery after effort, etc.
(2) A sensor of activity which is generally an accelerometer integrated into the pacemaker, denominated by the common name of “sensor G”, intended to quickly detect a change of the posture or of the dynamic of the patient carrying the apparatus, in particular to detect the beginnings of a phase of effort revealed by a significant increase in the patient's physical movements.
(3) A hemodynamic sensor: it is a question in this case of operating a control algorithm based upon a signal representative of the blood flow.
The effort sensor, also called a physiological sensor, delivers a signal that is well correlated to the real metabolic needs of the patient, but with a response time that is relatively long, and with a low sensitivity to low levels of effort.
Activity sensors on the other hand are sensors with a very short response time, but which measure a purely mechanical parameter (acceleration) that is non-physiological in nature, and therefore lacks specificity. Such a sensor does not allow, for example, to distinguish between a real beginning of an effort from vibrations or movements undergone in a purely passive way, for example traveling in a car, in which latter case the patient is not exerting any particular effort.
Rate responsive pacemakers are known that use one of these types of sensors to adjust permanently various parameters such as the stimulation frequency, the atrio-ventricular delay (AVD), or the inter-ventricular delay in the case of a bi-ventricular stimulation. There are also pacemakers combining two (or more) types of sensors, so as to avoid the disadvantages associated with each one.
The algorithms for controlling pacemakers in addition envisage a parameter known as the “maximum frequency” or “Fmax ”, which is the maximum frequency of ventricular stimulation. This parameter is applicable in particular when it is a question of ensuring the follow-up of the atrial rate by the ventricle: Fmax is then the higher limit to which the pacemaker can synchronize a ventricular stimulation on each atrial detection in the conventional DDD pacing mode. This Fmax parameter is in particular used to set a maximum limit for the stimulation frequency that may be calculated by algorithms such as the smoothing functions of rate response functions. In a rate responsive pacemaker, Fmax is used to make the dynamics of the sensor correspond to the upper limit value that the stimulation frequency can take.
In a double-chamber pacemaker, the maximum frequency also is used as a reference value, in comparison with the detected atrial frequency in order to limit the ventricular stimulation frequency when the atrial rate exceeds Fmax, for example, by applying an operating mode known as the “Wenckebach mode”.
The maximum frequency is generally programmed once at a value determined by the physician, mainly according to the age of the patient, with a possible weighting factor due to the capacity of effort of the patient and/or the presence of a cardiopathy or a cardiomyopathy.
It has been proposed to vary the maximum frequency in a way controlled over the course of time, as, for example, described in EP-A-1 059 099 and its corresponding Published U.S. Application 2000US-09589339 000607 (commonly assigned herewith to Ela Médical), where this frequency is automatically and gradually recorded over the course of time according to the hemodynamic improvement of the state of the patient.
A mechanism for the adjustment of the maximum frequency was also proposed by U.S. Pat. No. 6,119,040 (Chirife), which describes a pacemaker of the type controlled by an activity sensor (an accelerometer or similar component) included in the case of the pacemaker. To compensate for the fact that such a control sensor is not correlated with the metabolic needs for the patient, this document proposes to make variable the maximum frequency by adjusting the latter in an automatic way according to a physiological parameter. Thus, a significant increase of the stimulation frequency in response to a situation of activity detected by an accelerometer is allowed only if there is confirmation of a significant increase in the metabolic requirements. This makes it possible to make a little more specific the rate responsive function of the pacemaker, while adding to it a significant safety parameter. In this document, the physiological parameter used to regulate the maximum frequency is the ventricular pre-ejection period (PEP), namely the interval of time included between the detection of a beginning of cardiac cycle (spontaneous or stimulated) and the beginning of the ventricular ejection: during this interval of time, the ventricle contracts but its volume does not change (isovolumic contraction), only the pressure inside the ventricle increases. The PEP ends as soon as the aortic valve and the pulmonary valve open, which has as a consequence the ejection of blood in the arteries, with a correlative reduction of the volume of the ventricles, which continues until the end of the diastole.
According to this document, the PEP is evaluated utilizing an intracardiac measurement of bio-impedance: this parameter indeed gives a dynamic image of the contraction of the myocardium. The analysis of the variations of impedance makes it possible to characterize the evolution of the systolic and diastolic phases, and thus the duration of the PEP. An increase in the cardiac rate that would not be associated with a corresponding shortening of the PEP is regarded as inadequate or excessive compared to the physiological needs for the patient. This makes it possible to compensate or, at the very least, to limit the effects of the non-physiological character of the activity sensor used for the control of the pacemaker.
It has been appreciated by the present inventor that if one fixes the maximum frequency at a given value, pre-programmed, this adjustment does not take into account the general hemodynamic state of the patient, and even less his evolution over the course of time, for example, in the case of an improvement or, on the contrary, in the case of an aggravation, of this state. Indeed, if the heart rate is too high, the heart will not properly fill with blood in a satisfactory manner, and consequently the volume of ejection falls. It is thus the case in such situations of tachycardia or fibrillation, and also in the case of application of stimulation pulses at too high a frequency, due to a bad adjustment of the maximum frequency compared to the actual state of the patient.
There is thus a turning point frequency at which the benefit obtained by an increase in blood flow from an increase in the stimulation frequency is lost by the reduction in the volume of ejection. It is thus important not to exceed this turning point frequency, under a penalty of a reduction in the cardiac flow.